fix: drop jQuery from clients dashboard, fail fast on vanished click targets

clients.html: vanilla DOM + URLSearchParams (jQuery CDN removed), poll
backoff while the server is unreachable, inline errors instead of
alert(), two-click confirm for close, revoke stale screenshot blob
URLs, and only re-render tables when displayed data changes so DOM
identity stays stable for automation targeting the page.

extension: _elementPosition returns null instead of throwing when the
element was removed mid-command, and the click/hover flow turns that
into an immediate CLICK_FAILED instead of hanging into a 5s timeout.

Author: williamkapke

extension/modules/background-click.js

@@ -98,6 +98,10 @@ export async function hover(tab, { selector, xpath }, click = false) {
       }
 
       const bounds = await getFromContentScript(tabId, '_elementPosition', { id: elementResult.element.id });
+      if (!bounds) {
+        // Element was removed from the DOM mid-command (e.g. the page re-rendered)
+        throw new Error(`Element disappeared before the ${click ? 'click' : 'hover'} completed`);
+      }
       const actualTargetX = bounds.x + bounds.width / 2;
       const actualTargetY = bounds.y + bounds.height / 2;
       await getFromContentScript(tabId, '_moveMouseSVG', { x: actualTargetX, y: actualTargetY });

extension/page-helpers.js

@@ -320,7 +320,8 @@ const helpers = {
   },
   _elementPosition: ({ id }) => {
     const element = document.getElementById(id);
-    return element.getBoundingClientRect();
+    // Null when the element was removed mid-command (e.g. the page re-rendered)
+    return element ? element.getBoundingClientRect() : null;
   },
   _connectionStateChanged: ({ status, connected }) => {
     // Remove existing connection classes